The invention relates to a method of dewatering suspensions or the like slurry-like mixtures, in particular settlement slurries in a high pressure pressing zone formed by filter belts, after preliminary dewatering.
Screen belt presses are used in many cases for the dewatering of settlement slurries. Frequently however such presses do not achieve the solids contents as are required by the dump or disposal site management authorities, in regard to the depositing of the press cakes. In order to achieve a condition of dumpability therefore, use is increasingly made for slurry dewatering purposes of high pressure belt presses as are described for example in the present applicants' German patent specification No 2 720 178. In order to make the best possible use of the capacity of the relatively expensive high pressure belt presses, it is today part of the state of the art to dispose upstream of the actual high pressure portion a preliminary dewatering stage in the form of a strainer belt or a rotating screening drum; that arrangement provides that a large amount of filtrate is already separated off in a simple and inexpensive manner and the high pressure press is then only charged with the remaining, concentrated residue of the thin slurry.
High pressure belt presses achieve a solids content in the press cake which is about 5 to 10% higher than in the case of a dewatering operation on normal screen belt presses. However a prerequisite in that respect is that the settlement slurry to be dewatered is of adequate dewatering quality so that the pressing pressure which can be produced in high pressure belt presses can be applied at all. So that high pressure belt presses can also be used successfully in relation to heavy slurries, as for example in the case of biological overflow slurry which has only a very low degree of pressing stability, and already begins to flow under a low pressure loading as a result of its thixotropic and structure-viscose properties and is pressed through the meshes of the filter cloth, the conventional practice is for sawdust or peat litter to be mixed in with thin slurry of that kind, as structure-forming substances. The slurry is made more pressure-stable by virtue of that additive and can be dewatered by means of high pressing pressures to a dumpable consistency. The disadvantages of that method are that on the one hand large amounts of suitable additive substances must be obtained, transported to the settlement installation and stored there, with consequential effects on cost, and that on the other hand the amount of waste matter to be dumped is considerably increased by the additive substances. The addition of 10 kg of sawdust/m.sup.3 of thin slurry with 3% solid content increases the disposal amount by 33%.
It is also known for the amount of press cake to be reduced by a subsequently effected thermal drying operation, using direct or indirect heating. Although such driers produce a very substantial quantitative reduction, they do however represent an expensive capital investment factor and they involve very high operating costs. In addition, with the temperatures which occur in the material to be dried, a number of harmful substances which cause severe environmental pollution and extremely foul-smelling sulphur and albumin compounds are expelled, which must be separated out of the vapours or condensates from the driers, by means of highly expensive procedures. Those substances are generally recycled to the settlement installation and seriously encumber same with their high biological and chemical oxygen demand.
In consideration of that state of the art, the inventor set himself the aim of substantially reducing the residual amount which remains in settlement slurry dewatering, in an inexpensive and environment-friendly manner, while avoiding the above-described shortcomings.